This invention relates to the field of eddy current testing of materials. In particular, this invention is a single nest of eddy current probes which allow the inspector to test various sizes and types of materials without changing the eddy current probe.
The nondestructive, through-thickness inspection of components by eddy current methods requires that the eddy current skin depth be on the order of the thickness of the component. The ability to identify the source of an eddy current response (defect, thickness change, lift-off change, or conductivity change), also requires that the effective radius of the probe be equal to or greater than the skin depth.
In many applications, eddy current testers and testing methods require the use of ferrite cup core probes. These ferrite cup core probes are of various sizes and generally the radius of the cup core should vary with the thickness of the material under test. Vernon, et al., Ser. No. 294,621 filed Jan. 9, 1989 teaches an "Eddy Current Method to Measure Distance Between a Scanned Surface and a Subsurface Defect" which requires the operator to choose a ferrite cup core probe having a radius roughly 1.4 times the thickness of the material under test. In this application the radius is taken to be 1/3 the outside diameter of the ferrite core. Without controlling the radius of the probe, the distance between the scanned surface and a subsurface defect cannot be determined for arbitrary thicknesses.
Another prior art method for measuring electrical resistivity entitled "Eddy Current Method for Measuring Electrical Resistivity and Device for Providing Accurate Phase Selection", Vernon, et al., Ser. No. 294,622, filed Jan. 9, 1989 also requires the operator to select a probe having a radius which varies according to the thickness of the material to be tested. Measurement of defect depth in the absence of calibration standards also requires this range of probe size to skin depth. Consequently, when a set of components having different thicknesses are to be inspected or a single component has sections of different thicknesses (i.e., aircraft wing skins) the eddy current skin depth must be changed by changing the frequency; it is also necessary to physically replace one probe with one of a different size to maintain the optimum ratio of effective radius to skin depth.
Therefore, it is an object of this invention to eliminate the need to change probes during the testing operation.
It is a further object to teach a compound probe whereby the operator can change the effective radius by simply selecting a switch, or switches.
It is still another object to teach a device that is adaptable to software control, thus reducing the required operator training.
It is a further object of the instant invention to teach a single nested eddy current ferrite cup core probe capable of exhibiting a number of discrete probe radii.
These and other objects and advantages are provided by the present invention by teaching a compound multi-diameter probe consisting of concentric nests of ferrite cup core probes which can be individually activated either by operator action or software control.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.